Summary Imatinib was previously established to prevent and reverse Type 1 diabetes (T1D) in the NOD mouse model of disease by investigators at UCSF including Arthur Weiss and Jeffrey Bluestone. Related to those findings and due to the potent action of imatinib, a tyrosine kinase inhibitor, on immune cells, it is presently in clinical trials for new onset T1D. The required cellular targets of imatinib therapy have not yet been elucidated, which may impede successful clinical translation of this otherwise promising approach. In the NOD mouse, autoreactive B lymphocytes are critical drivers of disease through their interactions with islet-reactive T cells. This autoreactive T-B collaboration also remains our best predictor of future disease in humans, as it results in the production of autoantibody that is found in the serum of individuals at high risk for T1D. Because B lymphocytes act at this critical node in T1D progression, we hypothesized that they could be the critical mediator of imatinib?s protective effect. Using multiple models in which we established T1D in the presence or absence of B lymphocytes, we have determined that diabetes is never reversed by imatinib in the absence of B lymphocytes. When B lymphocytes are present, the effect of imatinib is both rapid and durable; diabetes is reversed in a matter of days and does not recur after treatment is stopped. When B lymphocytes are absent, there is no effect on blood sugar in any time frame. Based on the durable effect, we hypothesize that imatinib has induced regulation by B lymphocytes that controls immunity. In Aim 1 we explore this immune effect by determining which B lymphocytes are modulated by imatinib using mass cytometry to identify the therapeutic effect on novel B cell subsets including the effect on autoantigen-reactive cells. We further investigate the immune targets of these B lymphocytes and the mechanism of effect. Because of the extremely rapid effect that only occurs with B lymphocytes, we also hypothesize that imatinib induces an effect on islet alpha and beta cell functional recovery through its induced B lymphocyte functions. In Aim 2, we will explore this innovative connection between the immune system and islet cell recovery by investigating the B cell dependent therapeutic effect on islet cell mass, hormone secretion, and transcription factor expression. Overall, our approach will shift our understanding of the role of B lymphocytes in targeted therapy for T1D by defining their inducible capacity to promote restoration of immune tolerance while simultaneously enhancing islet cell functional recovery.